The investigation will focus on possible charges of causing involuntary injuries and endangering the lives of others but also possible cheating and failures in carrying out a product recall, the source said.
The investigation was first reported by the French news magazine Le Point.
Reports of around 20 children who had consumed Lactalis powdered milk -- sold under a range of brand names in France and abroad -- first emerged at the start of December.
The company, one of the world's largest producers of dairy products, ordered a first major recall on 10 December of nearly 7,000 tonnes of packets produced by a contaminated factory in Craon, northwest France.
At the time, it said it did not know how much of the potentially dangerous powder had been consumed or was in shops around the world and it announced a second, wider recall on 21 December.
The product withdrawals have affected consumers in countries such as China, Pakistan, Bangladesh, Britain and Sudan, underlining the reach of the company and the difficulty in tracing all the potentially at-risk powder.
Salmonella symptoms include severe diarrhoea, stomach cramps and vomiting. The illness -- caused by intestinal bacteria from farm animals -- is dangerous for the very young and elderly because it can cause severe dehydration.
A total of 35 children fell sick with salmonella poisoning in France since August, an unusually high number which led health authorities to refer to an epidemic of the illness.
16 children had to be treated in hospital and 31 were found to have consumed Lactalis products from its factory in Craon.
The father of a three-month-old baby who drank the milk, and the UFC Que Choisir consumer association, had filed a complaint against Lactalis.
The legal probe will be opened by prosecutors in Paris who are specialised in public health issues.
- with AFP